/Engine/Source/Runtime/Engine/Classes/Engine/StreamableManager.h
Crawls out to the outermost handle and returns it. Since we could have multiple Parent handles, we might have a tie. In cases of a tie, the eldest parent handle (so, lower indices) will win.
The single parent which is the most outer, or this if no Parent Handle
[TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [FStreamableHandle](API\Runtime\Engine\Engine\FStreamableHandle) > GetOutermostHandle()